The G-Dwarf Problem - Sixty Symbols

i'm going to talk about a truly excellent paper uh entitled sdss4 manga the g dwarf problem revisited did you write this paper as it happens i did yeah well i saw that coming to be fair it's actually the work of one of my phd students guy called mick greener and various of my colleagues around nottingham and other places as well but yes i'm one of the people involved in this excellent conflict of interest but we've dealt with it yeah absolutely no no i'm hugely conflicted but once in a while when you're doing astronomy you find something that nobody else ever knew before and you just get that kind of warm feeling about knowing something about the universe that nobody else knows it's it's why i do research oh there we go tell me about what's going on yeah so we need to talk about the g dwarf problem first um which is as with most things in astronomy a misnomer in that it's not really much to do with g-dwarfs and it's not really a problem and to talk about it we need to think a bit about star formation the simplest picture that you could have of star formation is you start with a big blob of gas and it just starts producing stars and some of them turn into stars and of course if you started out with just primordial gas are basically just hydrogen and helium from the universe that first generation of stars would just be hydrogen and helium but pretty quickly the most massive stars that get made will blow up a supernova they'll create some heavy elements from the nuclear processes that are going on within them and they'll start polluting the environment with heavier elements which means that the next generation of stars that form start having some heavier elements in them they'll then do the same thing enhance the heavy elements a bit more blow up the supernovae and so on and so you have a whole series of generations of stars each one slightly more heavy elements than the previous and in fact we don't find any stars we can't find that first generation of stars so few of them if they even still exist compared to everything that we find since that actually you don't find any with no metallicity no heavy elements at all so i will keep referring to metallicity that's astronomers like to annoy chemists by referring to everything heavier than helium as a metal and so metallicity just means the amount of heavy elements in something the other thing that can happen of course is if you just have this simple picture of a kind of isolated blob of gas so what's referred to as the closed box scenario where you've just got just this gas and nothing else because you're going to run out of gas and so you'll start making stars and mic more and more stars but then pretty quickly you'll have turned it all into stars and there won't be any more stars that get made one of the things you can do is say okay so in that scenario what kind of distribution of heavy elements would you expect would you expect to find lots of stars with little heavy elements and not many stars with lots of heavy elements or would you expect to find very few stars with not much by wave heavy elements and lots of stars with lots of heavy elements in and it turns out at least in that simple picture you can actually just do the calculation and the answer is that you would expect to see lots of stars without much by way of heavy elements in them and the reason why it works out that way is because you start turning this gas into stars and so those first generations of stars don't have much by way of heavy elements in them by the time you've got enough heavy elements made to make stars with lots of heavy elements in them there isn't much gas left and so you don't end up making very many stars that way so pictorially there's a way of showing this so here's a figure i was hoping for some really cool three-dimensional box with colors and gases but i've just no no very simple you know this is a very simple model so it's a very simple calculation and this is basically saying how much mass is there in stars that have less than some metals of d z as a function of that metallicity z and what this is saying is that in this closed box models of the blue line on here you start making lots and lots of stars with not much by way of heavy elements but then you don't add many more stars at heavier elements so you end up with a curve that looks like that unfortunately that's not what you see in the milky way if you look at the milky way and in particular the first place people looked at this was at g-dwarfs this is why it gets called the g-dwarf problem so the sun's a g-dwarf it's not specific to g-dwarf there are lots of g-dwarfs out there so they're easy stars to look at they are unevolved stars so they haven't kind of stirred up all their own insides they haven't generated lots of heavy elements of their own what you see on the surface of a g-dwarf is pretty much what that star was made of so they're good from that point of view and they're bright enough to see a reasonable distance away the picture we said is what we naively expect is you'd expect lots of things without much by way of heavy elements in them and not very many with lots of heavy elements in them yeah what you find in the milky way is completely the opposite so in general stars in the milky way are more polluted than you would have expected exactly so the simplest way it turns out to fix this is instead of having this thing called the closed box you can have a thing called an accreting box which is just basically you start out with a certain amount of gas you start turning it into stars but as time goes on you feed more gas in and it turns out that fixes the problem naively you might expect it would make things worse because you're feeding in primordial gas so this is gas which that hasn't been polluted at all you'd think that would actually end up making more stars with little by wear pollution but actually it turns out it has the opposite effect and the reason is because your clothes box is quite good at making the heavy elements so you've got lots of heavy elements around you just haven't got enough mass around to make more stars yeah and so the the accreted material just kind of gives you the extra material you need to start making those stars with more heavy elements in them more polluted stars so it's almost like your closed box later in life is saying i could make loads of polluted stars if you just give me a bit more gas to make stars with and it's like there we go the box is now full of fruit and currants it just hasn't got any flour to make more cakes with perfect yes so yes you've got all the you've got the fun ingredients but you just haven't got the basic raw material to bulk it up to make stars with and so that's what the the red line here shows is just what would happen if you have this accreting uh box model and it's you can see it's doing the opposite thing right the blue line where you didn't have that material you made lots of these stars with without much by way of pollution in them and very few got added later on here you can see you make fewer stars without much heavy elements in them but you've got plenty of material at the end to make lots of stars that have heavy elements in them and that solves the problem for the milky way right that actually if we say okay maybe the milky way wasn't one of these closed boxes we know it's not the closed box because we can see things smashing into it maybe just more and more material was getting added to it maybe that's the solution the neat part is we can now kind of do the same thing in external galaxies with this survey manga which i talked about a bunch of times before where basically we're taking spectra across the faces of galaxies we can play these clever games of actually splitting that light up and saying okay we can see the spectrum at each point in this galaxy how much of that spectrum looks like it comes from stars with high metallicities intermediate metalistic low metallicities so let me show you the same kind of plot that we had before but now for a whole bunch of galaxies and we've kind of combined galaxies together so the red line here is what we find for the most massive galaxies all the way up to the purple line which is what we find for the least massive galaxies all kind of average together in those different masses and what we find is that for for massive galaxies like the milky way we had exactly the same picture as for the milky way that you need this accreting box explanation because there aren't very many stars uh at low metallicity and there are lots and lots of stars at high metallicity so it looks like the milky way is absolutely typical for its kind there's nothing weird going on with the milky way the more interesting thing though is as we go down in mass and look at lower and lower mass galaxies when you look at the lowest mass galaxies which is the purple one here there they look very much like the closed box right that actually there are lots and lots of stars that are formed with these low metallicity so the curve goes up very quickly to start with and then we don't add very many more when we get to the higher metallicities so it looks like low mass galaxies really do behave as if they were kind of close boxes that you don't actually have to keep adding mass to a low mass galaxy to explain its distribution of metallicities whereas high mass galaxies like the milky way you have to keep adding more and more gas as the evolution's gone on surely that's exactly as you would have expected because a small galaxy is probably less likely to be able to pull in gas from other sources because it's got less of a presence but you know relative to its own size you know the milky way might pull in 10 percent of its mass because that's a big big lot of gas but a little galaxy will pull in you know also pull in 10 of its mass so you might expect this thing to kind of be scale invariant and they all ought to behave the same way but it clearly doesn't seem to be the case it really looks like the low-mass galaxies they've just had one load of gas and that was all they got and they turned that into stars whereas big galaxies like the milky way have been fed continuous stream of gas which has allowed them to make more and more higher metallicity stars although i talked about you know adding gas it could be that that gas was already in a big galaxy like the milky way it was already there it just wasn't forming stars it was just kind of hanging out in the outer parts of the galaxy and then you know as the star formation was doing its thing it would then start feeding it in whereas little galaxies it looks like all the gas is involved in star formation all the time which is another way of saying that it's much better mixed everything's all mixed up so that actually there isn't one load of gas over here which is not forming stars and a load of gas over here that is forming stars it's all all forming stars and all churning together professor i don't know if it's because we're in lockdown but at the moment when we're talking about this all i'm thinking about is like you know ingredients and and sultanas and flour and bakeries and things like that maybe i've made too many cakes in the last week or two when you lay in bed and think about this stuff you know are you i what scales and time scales do you think obviously you're not thinking about it on galactic time scales because you'd have to lay in bed for 100 000 years or something but like how do you imagine this stuff when you're lying in bed are you imagining all the graphs and the mathematics are you imagining the objects and things falling in but just in fast forward like what does that look like inside your head when you think about this when you look at one of the simulations on the screen right where you see the whole thing nice and small and in fast forward you see it in your head as happening on human time scales but then you just have to take that step back and say but actually that process is taking hundreds of millions of years not a few seconds to be honest the the main time i stop and get awed by it is when i talk to you or you know when i give a popular talk or when i'm trying to explain it and suddenly you do have to take that step back and it's no longer you know that one with all those zeroes that i just take for granted suddenly you've got a whole bunch of people saying well that's an amazingly big number and then you have to take that actually you're quite right it is an amazingly big number there's also something about it though when you think about the reality of the size and more the time scale while it is more awesome it sort of becomes more boring is that fair like it's almost like you think well yeah that looks cool in a simulation but in fact that's just happening at something that beyond glacial like it's all just happening so slowly isn't that amazing in itself i think that's amazing that this this majestic scale of this thing right it's not you know it's a huge symphony that's going on it's not some little tune and actually the time it takes to unfold is enormously long i think that adds to the beauty of it not detracts from it it's all right if you've got long enough to sit around and watch it but come on we're in a hurry here but that's the neat thing right isn't it amazing that we can actually start to understand things which take hundreds of millions of years to happen it's quite amazing that a human mind which as you say is kind of focused on things happening on time scales of hours can actually comprehend things which take billions of years in some cases i think that's pretty amazing really they use the super protons synchrotron collider at cern they had a beam of electrons of about 100 gv of these lego bricks represent 100 kilometers on this map to put some things into scale first let's talk about airliners so when we're on a commercial airliner

Channel: Sixty Symbols

Views: 95,568

Keywords: sixtysymbols, stars, astronomy

Id: nhNPKHbg_64

Length: 11min 19sec (679 seconds)

Published: Thu Feb 04 2021